The replacement cost of tires is one of the major expenses encountered by the trucking industry. Tire replacement cost and frequency is, of course, also of concern to most automobile and light truck owners. In recent years, many modifications have been implemented to improve the treadwear characteristics of tires. However, improvements in tire treadwear characteristics have sometimes been achieved by compromising the traction and/or rolling resistance characteristics of the tire.
In order to reduce the rolling resistance of a tire, rubbers having a high rebound can be utilized in making the tires treads. Tires made with such rubbers undergo less energy loss during rolling and normally also exhibit improved treadwear characteristics. The traditional problem associated with this approach is that the tire's wet traction and wet skid resistance characteristics are compromised. This is because good rolling resistance which favors low energy loss and good traction characteristics which favor high energy loss are viscoelastically inconsistent properties.
In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber and polybutadiene rubber are commonly used as a rubbery material for automobile tire treads. However, such blends are not totally satisfactory for all purposes.
U.S. Pat. No. 5,137,998 discloses a process for preparing a rubbery terpolymer of styrene, isoprene and butadiene having multiple glass transition temperatures and having an excellent combination of properties for use in making tire treads which comprises terpolymerizing styrene, isoprene and 1,3-butadiene in an organic solvent at a temperature of no more than about 40.degree. C. in the presence of (a) at least one member selected from the group consisting of tripiperidino phosphine oxide and alkali metal alkoxides and (b) an organolithium compound.
U.S. Pat. No. 5,047,483 discloses a pneumatic tire having an outer circumferential tread where said tread is a sulfur-cured rubber composition comprised of, based on 100 parts by weight rubber (phr), (A) about 10 to about 90 parts by weight of a styrene, isoprene, butadiene terpolymer rubber (SIBR), and (B) about 70 to about 30 weight percent of at least one of cis 1,4-polyisoprene rubber and cis 1,4-polybutadiene rubber wherein said SIBR rubber is comprised of (1) about 10 to about 35 weight percent bound styrene, (2) about 30 to about 50 weight percent bound isoprene and (3) about 30 to about 40 weight percent bound butadiene and is characterized by having a single glass transition temperature (Tg) which is in the range of about -10.degree. C. to about -40.degree. C. and, further the said bound butadiene structure contains about 30 to about 40 percent 1,2-vinyl units, the said bound isoprene structure contains about 10 to about 30 percent 3,4-units, and the sum of the percent 1,2-vinyl units of the bound butadiene and the percent 3,4-units of the bound isoprene is in the range of about 40 to about 70 percent.
U.S. Pat. No. 5,272,220 discloses a styrene-isoprene-butadiene rubber which is particularly valuable for use in making truck tire treads which exhibit improved rolling resistance and tread wear characteristics, said rubber being comprised of repeat units which are derived from about 5 weight percent to about 20 weight percent styrene, from about 7 weight percent to about 35 weight percent isoprene, and from about 55 weight percent to about 88 weight percent 1,3-butadiene, wherein the repeat units derived from styrene, isoprene and 1,3-butadiene are in essentially random order, wherein from about 25 percent to about 40 percent of the repeat units derived from the 1,3-butadiene are of the cis-microstructure, wherein from about 40 percent to about 60 percent of the repeat units derived from the 1,3-butadiene are of the trans-microstructure, wherein from about 5 percent to about 25 percent of the repeat units derived from the 1,3-butadiene are of the vinyl-microstructure, wherein from about 75 percent to about 90 percent of the repeat units derived from the isoprene are of the 1,4-microstructure, wherein from about 10 percent to about 25 percent of the repeat units derived from the isoprene are of the 3,4-microstructure, wherein the rubber has a glass transition temperature which is within the range of about -90.degree. C. to about -70.degree. C., wherein the rubber has a number average molecular weight which is within the range of about 150,000 to about 400,000, wherein the rubber has a weight average molecular weight of about 300,000 to about 800,000, and wherein the rubber has an inhomogeneity which is within the range of about 0.5 to about 1.5.
U.S. Pat. No. 5,239,009 reveals a process for preparing a rubbery polymer which comprises: (a) polymerizing a conjugated diene monomer with a lithium initiator in the substantial absence of polar modifiers at a temperature which is within the range of about 5.degree. C. to about 100.degree. C. to produce a living polydiene segment having a number average molecular weight which is within the range of about 25,000 to about 350,000 and (b) utilizing the living polydiene segment to initiate the terpolymerization of 1,3-butadiene, isoprene and styrene, wherein the terpolymerization is conducted in the presence of at least one polar modifier at a temperature which is within the range of about 5.degree. C. to about 70.degree. C. to produce a final segment which is comprised of repeat units which are derived from 1,3-butadiene, isoprene and styrene, wherein the final segment has a number average molecular weight which is within the range of about 25,000 to about 350,000. The rubbery polymer made by this process is reported to be useful for improving the wet skid resistance and traction characteristics of tires without sacrificing tread wear or rolling resistance.
U.S. Pat. No. 5,061,765 discloses isoprene-butadiene copolymers having high vinyl contents which can reportedly be employed in building tires which have improved traction, rolling resistance and abrasion resistance. These high vinyl isoprene-butadiene rubbers are synthesized by copolymerizing 1,3-butadiene monomer and isoprene monomer in an organic solvent at a temperature which is within the range of about -10.degree. C. to about 100.degree. C. in the presence of a catalyst system which is comprised of (a) an organoiron compound, (b) an organoaluminum compound, (c) a chelating aromatic amine and (d) a protonic compound; wherein the molar ratio of the chelating amine to the organoiron compound is within the range of about 0.1:1 to about 1:1, wherein the molar ratio of the organoaluminum compound to the organoiron compound is within the range of about 5:1 to about 200:1 and wherein the molar ratio of the protonic compound to the organoaluminum compound is within the range of about 0.001:1 to about 0.2:1.
U.S. Pat. No. 5,405,927 discloses an isoprene-butadiene rubber which is particularly valuable for use in making truck tire treads, said rubber being comprised of repeat units which are derived from about 20 weight percent to about 50 weight percent isoprene and from about 50 weight percent to about 80 weight percent 1,3-butadiene, wherein the repeat units derived from isoprene and 1,3-butadiene are in essentially random order, wherein from about 3 percent to about 10 percent of the repeat units in said rubber are 1,2-polybutadiene units, wherein from about 50 percent to about 70 percent of the repeat units in said rubber are 1,4-polybutadiene units, wherein from about 1 percent to about 4 percent of the repeat units in said rubber are 3,4-polyisoprene units, wherein from about 25 percent to about 40 percent of the repeat units in the polymer are 1,4-polyisoprene units, wherein the rubber has a glass transition temperature which is within the range of about -90.degree. C. to about -75.degree. C. and wherein the rubber has a Mooney viscosity which is within the range of about 55 to about 140.
Cold flow is a problem that is commonly encountered in the handling and storage of many low molecular weight linear rubbers utilized in making tire tread compounds. For instance, cold flow is a particular problem with linear polymers having low glass transition temperatures and low molecular weights. Cold flow often presents a problem in isoprene-butadiene rubbers, low vinyl polybutadiene rubbers, medium vinyl polybutadiene rubbers and polyisoprene rubbers having low molecular weights and low glass transition temperatures which are used in making tire treads. These polymers are often crosslinked with divinyl benzene to reduce cold flow. However, this technique for overcoming cold flow can lead to other problems. For example, gelation can occur and polymer inconsistency often results. The crosslinking can also detrimentally change the physical characteristics of the rubbery polymer.